The following are known as state of the art:
1. Components of superconductivity electronics
Epitactic layer sequences or multilayer systems for such components are comprised of one or more thin films of a superconductive material and one or more thin films of nonsuperconductive material. These nonsuperconductive materials are effective as barrier materials in Josephson contacts or junctions, for passivation or as a diffusion block. Based on the characteristics of the high-temperature superconductor, the following are the requirements for the nonsuperconductors:
The high temperature superconductive and the nonsuperconductive materials must be chemically compatible. This means that no chemical reactions should occur between the materials. The nonsuperconductive material should be able to grow epitaxially on the high-temperature super-conductive material and the high-temperature superconductive material should be correspondingly able to grow on the nonsuperconductive material with, indeed, the desired crystallographic orientation. The thus-resulting boundary interfaces should be atomically sharp and should not contain any defect-oriented regions or extraneous phases in their environs. Because of the relatively high fabrication temperatures of the layers, an interdiffusion of ions cannot be excluded and hence it must be ensured that any extraneous ions which are in the material affect the properties thereof to the smallest extent possible. It is, for example, known that above all small ions like those of Al, Ga, Ti, W, Fe, and Zn, or even Ce, Pr, reduce the superconductivity of the high-temperature superconductor REBa.sub.2 CU.sub.3 O.sub.7-z (RE=rare earth element).
This applies inter alia also to the oxygen content and the ordering of the oxygen atoms in the high-temperature superconductors in which the superconductivity is weakened by oxygen loss or oxygen disordering. A high degree of chemical compatibility is required when the nonsuperconductive material is used, for example, as thin barriers in Josephson junctions. Because of the reduced coherency lengths of high-temperature superconductors--typically in the range of 1 nm to 2 nm--it is required that the barrier material not have its coherency length, which is in the length range of the ordering parameter of the superconductive electrodes in the proximity of the boundary interface, reduced for example by ion diffusion or lattice dislocation. Up to now no material has been known which fulfills this requirement satisfactorily.
Materials research has been concentrated on two material classes. One class is oriented upon the structure of REBa.sub.2 Cu.sub.3 O.sub.7-z. The research here is in targeted replacement and doping with one or more ion types which reduce the superconductive properties or even completely suppress them. The second class encompasses Perovskite or Perovskite-like compounds. From each class at an appropriate location, one material will be discussed.
The nonsuperconductive material PrBa.sub.2 Cu.sub.3 O.sub.7-z differs chemically from YBa.sub.2 Cu.sub.3 O.sub.7-z only by the substitution of Y by Pr which effects the loss of suuperconductivity. The lattice-defect match with YBa.sub.2 Cu.sub.3 O.sub.7-z amounts to only 1%. By comparison to most of the other, hitherto researched, nonsuperconductive materials, PrBa.sub.2 Cu.sub.3 O.sub.7-z has the highest degree of chemical and structural compatibility with YBa.sub.2 Cu.sub.3 O.sub.7-z. For example, a monolayer of YBa.sub.2 Cu.sub.3 O.sub.7-z which serves as the intermediate layer in a PrBa.sub.2 Cu.sub.3 O.sub.7-z matrix, has a critical temperature {T.sub.c } of 30 K (T. Terashima et al, Phys. Rev. Lett. 67, 1362 (1991)).
Similar experiments with other nonsuperconductive materials show that these values cannot be attained with any other material. Substantial disadvantages of PrBa.sub.2 Cu.sub.3 O.sub.7-z are, however, its relatively low specific resistance, which makes it less than satisfactory for insulation purposes, and the reduction of the ordering parameter at the boundary interface as a result of diffusion of Pr ions into the YBa.sub.2 Cu.sub.3 O.sub.7-z. If one replaces for example only 5% of the Y atoms in YBa.sub.2 Cu.sub.3 O.sub.7-z by Pr atoms, the critical temperature is already reduced from 92 K to 85 K (M. S. Hedge, et al., Phys. Rev. B 48, 6465 (1993)).
A typical representative of the second class of Perovskite-like compounds is SrTiO.sub.3. This material has a cubic crystal structure whose lattice-defect matching to YBa.sub.2 Cu.sub.3 O.sub.7-z amounts to 1.2%. The specific resistance at 200 bMcm is clearly greater than that for PrBa.sub.2 Cu.sub.3 O.sub.7-z. It shows that with this material a heteroepitaxy is possible with YBa.sub.2 Cu.sub.3 O.sub.7-z. The chemical compatibility of the materials is, however, limited. The diffusion of Ti ions as well as their incorporation in the Cu sites of YBa.sub.2 Cu.sub.3 O.sub.7-z gives rise to a reduction of the ordering parameter in the vicinity of the boundary interface. Furthermore, the lattice distortion in the YBa.sub.2 Cu.sub.3 O.sub.7-z resulting from the boundary interface with the SrTiO.sub.3, reduces the ordering parameter in a noticeable manner.
2. Buffer layers
Application-oriented requirements can make it desirable to grow high-temperature superconductor thin layers or a component containing such a layer on a substrate which is not suitable, e.g. from the aspect of chemical compatibility. Examples of this are the materials silicon and sapphire. Both react in an undesired manner chemically with the high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.7-z.
To form an epitaxy on these substrates, one or more so-called buffer layers are introduced which are disposed between the substrate and the thin layer/thin-layer system. Buffer layers are used to generate smoother surfaces of the high- temperature superconductor layer on certain substrates.
For SrTiO.sub.3 buffer layers a so-called leveling effect is observed. That means that, when SrTiO.sub.3 is grown on an atomic stage containing YBa.sub.2 Cu.sub.3 O.sub.7-z surfaces, it covers these surfaces and forms after several nm in thickness, a smooth [100] surface. This surface is then suitable for the c-axis-oriented growth of YBa.sub.2 Cu.sub.3 O.sub.7-z. A buffer layer serves in general the purpose of improving certain characteristics of a thin layer on a given substrate.
The requirements as to the quality of buffer layers are comparable to those of the nonsuperconductive layers for components. It is conceivable to take into consideration certain drawbacks of a buffer layer. For instance a local reduction of the ordering parameter at the boundary interface between substrate at high temperature superconductor can be expected to a certain extent when the layer thickness of the superconductor is greater than, for example, 30 nm.
For silicon, for example, yttrium-stabilized zircon (YSZ) can be used as a buffer layer. The lattice-defect matching of YSZ is relatively high at up to 6%. The chemical compatibility is only limited. It has been observed that at the boundary interface BaZrO.sub.3 develops which, as an extraneous phase, reduces the ordering parameter of the YBa.sub.2 Cu.sub.3 O.sub.7-z. In addition, there is a diffusion of Zr to the Cu sites with the result that a reduced ordering parameter is communicated to the superconductor.
If YBa.sub.2 Cu.sub.3 O.sub.7-z is directly sputtered onto a sapphire substrate, BaAl.sub.2 O.sub.4 can arise at the boundary interface and which highly interferes with the epitaxy of the YBa.sub.2 Cu.sub.3 O.sub.7-z growing thereon (K. Dovidenko, S. Oktyabrsky and A. Ivanov, Mater. Si. Eng. B 15, 25 (1992)). For sapphire substrates, CeO.sub.2, inter alia, has been used as a buffer. In this case CeO.sub.2 can grow in two different orientations (A.G. Zaitsev, R. Kutzner, R. Wdrdenweber, Appl. Phys. Lett. 67, 1 (1995)) which, as a consequence, is clearly detrimental to the epitaxy of the YBa.sub.2 Cu.sub.3 O.sub.7-z layer deposited on the CeO.sub.2.
3. Substrate Materials
The use of materials as substrates for epitactic high-temperature superconductor thin films has as a precondition the ability to fabricate it as a macroscopic monocrystal. Further, for a sufficient quality, the substrate material in question must satisfy the following requirements:
chemical compatibility with the thin-layer material to be grown PA1 quality of the surface PA1 purity of the material PA1 homogeneity of the substrate PA1 thermodynamic stability PA1 low lattice-defect matching to the thin-layer PA1 low difference in the thermal coefficient of expansion. PA1 (i) BaTbO.sub.3 PA1 (ii) Ba.sub.1-x Sr.sub.x TbO.sub.3 with O.ltoreq.x.ltoreq.1; PA1 (iii) LaCu.sub.1-x Tb.sub.x O.sub.3 with O.ltoreq.x.ltoreq.1; PA1 (iv) Rcu.sub.1-x Tb.sub.x O.sub.3 with R=Nd, Eu, Sm and O.ltoreq.x.ltoreq.1; PA1 (v) R.sub.1-y N.sub.y Cu.sub.1-x Tb.sub.x O.sub.3 with R=La, Nd, Eu, Sm; PA1 (vi) R.sub.2-y N.sub.y Cu.sub.1-x Tb.sub.x O.sub.4 with R=La, Nd, Eu, Sm; PA1 (vii) A.sup.1.sub.1-x A.sup.2.sub.x B.sup.1.sub.1-y B.sup.2.sub.y O.sub.3 with A.sup.1 =Ba, Sr; PA1 small lattice defect match for thin film material; PA1 quality of the surface; PA1 purity of the material; PA1 homogeneity of the substrate; PA1 thermal dynamic stability; PA1 reduced difference in the thermal coefficient of expansion; and PA1 chemical compatibility with the thin film material to be grown. PA1 (i) BaTbO.sub.3 PA1 (ii) Ba.sub.1-x Sr.sub.x TbO.sub.3 with O.ltoreq.x.ltoreq.1 PA1 (iii) LaCu.sub.1-x Tb.sub.x O.sub.3 with O.ltoreq.x.ltoreq.1 PA1 (iv) RCu.sub.1-x Tb.sub.x O.sub.3 with R=Nd, Eu, Sm and O.ltoreq.x.ltoreq.1; PA1 (v) Ba.sub.1-x Sr.sub.x MO.sub.3 with M=Tb, Pr, Ce and O.ltoreq.x.ltoreq.1; PA1 (vi) LaCu.sub.1-x M.sub.x O.sub.3 with Tb, Pr, Ce and O.ltoreq.x.ltoreq.1; PA1 (vii) RCu.sub.1-x M.sub.x O.sub.3 with R=Nd, Eu, Sm PA1 (viii) R.sub.1-y N.sub.y Cu.sub.1-x M.sub.x O.sub.3 with R=La, Nd, Eu, Sm; PA1 (ix) R.sub.2-y N.sub.y Cu.sub.1-x M.sub.x O.sub.4 with R=La, Nd, Eu, Sm; PA1 (x) A.sup.1.sub.1-x A.sup.2.sub.x B.sup.1.sub.1-y B.sup.2.sub.y O.sub.3
Of these, the requirement for chemical compatibility has the highest priority, since most substrate materials contain ions which can be detrimental to a high-temperature superconductor upon interdiffusion therewith. An interdiffusion cannot, however, be excluded because of the relatively high fabrication temperature of the high-temperature superconductor. Thus known substrate materials like SrTiO.sub.3, LaAlO.sub.3 and MgO can contain ions like Ti.sup.+4, Al.sup.+3 and Mg.sup.+2 which, in the superconductor, especially in YBa.sub.2 Cu.sub.3 O.sub.7-z, substantially reduce the critical temperature.
In summary, the following problem fields have been discerned in which play a role in the three discussed use fields as superconductive components, as buffer layers, and as substrates.
All materials used hitherto in Josephson junctions as barriers show only a limited degree of chemical compatibility with the high-temperature superconductors whereby the obtained characteristics of the Josephson junction remain below what could theoretically be expected.
Furthermore, there are no known buffer materials upon which an ultrathin YBa.sub.2 Cu.sub.3 O.sub.7-z layer can be superconductively formed and whereby the buffer layer simultaneously forms an atomic stage with a leveling effect.
For the three planned fields named, materials have most frequently been used which are only limitedly chemically compatible with the high-temperature superconductors. This affects detrimentally the superconductive layers grown thereon, and whose superconductive characteristics, especially with reduced thickness of the superconductive layer, are degraded.